1. Field of the Invention
The present invention relates to a microstrip switch for use in RF switch applications and more particularly to a broadband radially combined single pole N-throw microstrip switch with an improved insertion loss characteristic at millimeter wave frequencies, formed with a low loss air suspended radially combined patch which reduces the parasitic shunt capacitance and thus extends the low-pass response of the device.
2. Description of the Prior Art
Microstrip switches are used in various RF applications. Various configurations for such microstrip switches are known. For example, cascaded switches and orthogonal arms switch configurations are known. Radial combined switches are also known which offer symmetrical switch arm performance and the consolidation of multiple switch arms in a relatively small area compared to cascaded switches and orthogonal arm switches. The relatively small size of radially combined switches is especially attractive for low cost, high volume applications, such as automotive radar.
There is a limit to the bandwidth and low insertion loss which can be achieved by a radially combined microstrip switch. More particularly, two fundamental performance limitations exist at high frequencies: the cutoff frequency performance of the semiconductor switch; and the effective low pass characteristics of the microstrip radially combined microstrip switch. Both of these factors degrade the performance of the radially combined microstrip switch to a greater extend as the number of switch arms is increased.
An exemplary N-way radially combined single pole N-throw microstrip switch, generally identified with the reference numeral 20, is illustrated in FIG. 1. As shown, the microstrip switch 20 includes N input switch arms, identified in FIG. 1 with the reference numerals 22-36, and an output switch arm 38. The input switch arms 22-36 and the output switch arm 38 are connected at a radial combined patch 40. Each input switch arm 22-36 includes a pair of serially coupled p-i-n diodes 40 and 42, connected between an input microstrip transmission line 44, which acts as an input port, and an interconnecting microstrip transmission line 46 for each of the input switch arms 22-36. The interconnecting microstrip transmission lines 46 for each of the input switch arms 22-36 are coupled together at the radially combined patch 40. A microstrip transmission line 48 is connected to the radial combined patch 40 to provide an output port for the switch. The input and output microstrip 44 and 48 are illustrated as being 50 .OMEGA..
Given the specific p-i-n diode process technology, the effect of gain-bandwidth or low loss bandwidth tradeoff of the microstrip switch 20 is adjusted by either scaling the size of the p-i-n diodes 40, 42 or by adding multiple p-i-n diodes in series, parallel or combinations thereof for each of the input switch arms 22-36. For high frequency operation, the p-i-n diodes 40,42 for each of the input switch arms 22-36 is configured such that the low pass cutoff frequency of the diodes 40,42 is beyond the operating frequency of interest.
FIGS. 2b and 2c represent the equivalent circuit models of a typical 2-.mu.m i-region GaAs p-i-n diode with a cutoff frequency with f.sub.c &gt;2 THz for a p-i-n diode of a particular size as illustrated in FIG. 2a. As shown in FIG. 2c, the series off capacitance is relatively substantial. In order to reduce series off capacitance, two p-i-n diodes in series may be utilized in order to extend the bandwidth response at the expense of insertion loss. Because of the use of 2-.mu.m GaAs p-i-n diodes with cutoff frequencies f.sub.c &gt;2 Thz and the relatively small size of the p-i-n diodes, the individual input switch arms 22-36 of the radially combined microstrip switch 20 will have a frequency response beyond the frequency of interest. It is the low pass roll-characteristic of the radially combined microstrip switch which will be the limiting performance factor for an N-way microstrip switch at millimeter-wave frequencies.
In general, for a large number of radially combined input switch arms 22-36, the radially combined patch 40 will be of significant area and will contribute to the dominant low-pass loss characteristics of the N-way switch 20 at millimeter-wave frequencies. By reducing the size of the radial combined patch 40, the associated parasitic impedances can be minimized and the frequency response extended. However, the width of the output 50 .OMEGA. microstrip transmission line 38, for example 70 .mu.m for a 4 mil GaAs substrate, will ultimately limit how small the radial combined patch 40 can be made as generally illustrated in FIG. 9.
FIG. 3 illustrates a lumped element equivalent circuit of the single pole N-throw radial combined microstrip switch 20 illustrated in FIG. 1. As shown, the radially combined patch 40 can be represented by a L-C low pass network 41. When the Nth input switch arm 36 is switched on and all of the other N-1 inputs switch arms 22-34 are switched off, the thru-path of the Nth input switch arm 36 can be represented by the equivalent circuit illustrated in FIG. 4. As shown in FIG. 4, the low pass response of the microstrip switch 20 can be characterized by a simple low pass filter network formed from a series inductance L.sub.feed and the effective parallel combination of the shunt capacitors C.sub.off (N-1)/2 and C.sub.feed. For very high performance Schottky p-i-n diodes with cutoff frequencies f.sub.c &gt;2 THz, the shunt capacitant C.sub.feed can typically account for .gtoreq.15% of the total effective shunt capacitance. When the radially combined patch 40 is large in diameter to accommodate a typical wide 50 .OMEGA. fixed output microstrip transmission line 38, the shunt capacitance C.sub.feed is large and the associated series inductance L.sub.feed is small. If the electrical and physical restraints allow the reduction of the diameter of the radially combined patch 40, the shunt capacitance of the radially combined patch 40 will become relatively smaller; however, the input microstrip transmission lines 44 will become more inductive. Thus, there is only a marginal benefit gained by changing the size in geometry of the radially combined patch, since the low pass pole, determined by series inductance L.sub.feed and shunt capacitance C.sub.feed, will not significantly change. Thus, enhanced frequency performance of a radially combined microstrip switch has not heretofore been known to be obtained by simply changing the size of the radially combined patch.